The present invention relates generally to a motor controller, and more particularly to a controller for controlling the rotational speed and armature current of a brushless DC motor.
Control circuits are known for controlling brushless DC motors, such as, for example, regulating the rotational speed of brushless DC fan motors that cool the interiors of computers. One problem with brushless DC fan motors is that they traditionally have had a narrow usable input range. Fan speed and input current are approximately proportional to input voltages. Thus, if the input voltage from an unregulated source such as a battery were used to power a brushless DC fan, such as a typical 24 volt nominal battery, the voltage would vary from about 28 volts in float state to about 21 volts in discharged state. This change would cause a brushless DC fan rated at a nominal 3500 RPM to vary as much as about 1000 RPM over the above-mentioned range of battery voltages. Such a large variation in RPM means that the fan is not properly cooling a computer at the low-end of the RPM range, and that power is being wasted at the high-end of the RPM range.
Some brushless DC fan users have multiple input source voltages that their equipment is expected to operate from, with 24 volt and 48 volt systems being the most common. Such multiple source voltages pose the same problem in resultant RPM variation in a brushless DC fan motor as does a single input voltage source whose voltage level varies widely. Accordingly, there is a need to provide a brushless DC fan motor having a high input range with relatively little variation in motor rotational speed. For example, in the telecommunications industry, there is a need to provide a brushless DC fan motor having an input range of about 20-60 volts with little variation in motor rotational speed. However, other input voltage ranges may be provided for other motor applications.
Linear regulators have been used to regulate brushless DC fan rotational speed. However, the linear regulator approach poses an efficiency problem. A brushless DC fan that draws 18 watts at 21 volts will draw almost 27 watts when operating at 28 volts, and 54 watts at 56 volts input, with the increase in power draw having to be dissipated as heat.
Pulse width modulation (xe2x80x9cpwmxe2x80x9d) has also been used in the prior art to regulate motor speed. One method commonly used is to pulse width modulate the commutation transistors to the brushless DC motor. This employment of pulse width modulation reduces the dissipation of energy involved with changing motor speed. However, pulse width modulating the commutation transistors does not permit large changes in input voltage without widely varying the rotational speed of the brushless DC motor. This method is most commonly used in thermal brushless DC fans to reduce brushless DC fan speed at low temperatures. The speed variation is unfortunately even wider than that of the non-speed controlled type, and clamp dissipation is still relatively high.
Another pwm approach is to use a fill bridge driver. This involves placing a bipolar motor winding between the legs of four switching transistors and controlling the timing of the pwm modulator and commutation logic to regulate motor current. Wide input voltage ranges are possible with high efficiency. A well designed full bridge driver can regulate motor speed over a better than 3:1 range of input voltage. The chief drawbacks are complicated logic and the difficulties of driving the four switching transistors without cross conduction through the series connected pairs. Although many manufacturers offer integrated full bridge devices, most suffer from a limitation of current and/or voltage.
Another approach is to employ a pwm switching voltage regulator to accommodate a wide range of input voltages without widely varying the rotational speed of the motor. However, this requires relatively bulky filter inductors and capacitors.
Of the above-mentioned pwm approaches, the pwm voltage regulator regulates motor voltage. The other methods typically regulate motor current. Voltage regulation is preferred to minimize variations in desired brushless DC motor speed. In other words, the variation in motor speed from motor to motor for a given current is greater than the variation in motor speed for a given voltage. Additionally, motor torque is a function of motor current. Therefore, if motor current is reduced in order to reduce motor speed to a low value, the motor torque becomes low. This means that the motor speed is sensitive to applied load (i.e., back pressure). This sensitivity to back pressure results in large speed deviations from the desired value. Motor-starting at low desired speeds is also a problem in that if the motor current is set too low then the motor will not be able to overcome the magnetic detents used to position the rotor away from the null point. Unfortunately, controlling motor voltage while failing to control motor current to adhere to a symmetrical waveform has the potential to increase vibration and electrical interference.
Fans typically use one of two types of two-phase DC brushless motors, unipolar or bipolar. The difference between the two types is that a unipolar motor energizes two opposing poles of the four poles available, whereas a bipolar motor will energize all four poles at the same time, with the coils in quadrature having opposite magnetic polarity. Simply stated the unipolar type uses two pairs of coils with one pair energized and the other pair not energized, with the poles always energized in the same polarity. The bipolar motor energizes the four poles at the same time with adjacent poles having opposite polarity. Rotation of the motor of the unipolar type is accomplished by alternating energized pairs while the bipolar motor changes the polarity of the four poles.
The bipolar motor has double the output of the unipolar motor because all of the copper is utilized and all four poles act upon the magnet. Drive complexity is greater as the direction of current must be reversed rather than just interrupted. In both cases however a problem of asymmetrical current in the motor exists. The current in the motor windings is reversed twice for each complete revolution of the bipolar motor. Various factors influence or modify the symmetry of the motor such as the degree of magnet strength, offset in the position sensor, mechanical variations in the motor components, and variations in wire resistance. This causes the current levels and the waveform shapes to differ from each other within a rotational period and allow different torques to be applied to the rotor, increasing vibration and noise. Accordingly, it would be desirable to provide an apparatus and method which may correct such non-ideal behavior in both unipolar and bipolar motors.
It is also an object of the present invention to provide a brushless DC motor regulator which handles a relatively wide range of input voltages with little variation in the rotational speed of the motor.
It is another object of the present invention to provide a brushless DC motor regulator which controls motor armature current to a substantially symmetrical waveform.
It is a further object of the present invention to provide a brushless DC motor regulator that eliminates the relatively bulky filter capacitors and inductors interfacing the regulator and motor.
The above and other objects and advantages of the present invention will become more readily apparent when the following detailed description is read in conjunction with the accompanying drawings.
According to one aspect of the present invention a control circuit for controlling the rotational speed of a brushless DC motor is provided. The control circuit includes an electrical conduction switch having an input, an output, and a control terminal for passing a motor supply signal to a brushless DC motor from a voltage across first and second terminals of a DC voltage source. The input terminal of the switch is to be coupled to the first terminal of the DC voltage source, and the output terminal of the switch is to be coupled to the first terminal of the brushless DC motor. A voltage averaging circuit is provided having first and second input terminals and an output terminal for averaging the voltage level of the motor supply signal. The first input terminal of the voltage averaging circuit is coupled to the output of the switch, and the second terminal of the voltage averaging circuit is to be coupled to the second terminal of the voltage source. A differential amplifier has first and second input terminals and an output terminal for generating a signal corresponding to motor speed error. The first input terminal of the differential amplifier is coupled to a voltage reference potential indicative of the desired motor speed, and the second input terminal of the differential amplifier is coupled to the output terminal of the voltage averaging circuit.
A pulse width modulator (xe2x80x9cpwmxe2x80x9d) of the control circuit has first and second input terminals and an output terminal. The first input terminal of the pwm is coupled to the output terminal of the differential amplifier for receiving the signal corresponding to motor speed error, the second input terminal of the pwm is coupled to a signal corresponding to the change in motor current, and the output terminal of the pwm is coupled to the control terminal of the electrical conduction switch. The pwm turns the switch on at a periodic rate, and turns the switch off after a delay, or pulse width, indicative of the difference in voltage level between the signal corresponding to motor speed error and the signal corresponding to change in motor current, in order to provide a motor supply signal having a substantially constant average voltage level corresponding with the desired motor speed and a substantially symmetrical current waveform. Preferably, the motor windings serve as an inductive filter to help smooth changes in current, and the rotor mass of the motor serves to help smooth the rotational speed of the motor.
According to another aspect of the present invention, a control circuit for controlling the rotational speed of a brushless DC motor is provided. The control circuit includes first means to be coupled to an electrical power source for switchably passing a motor supply signal to a brushless DC motor. A second means is coupled to an output of the first means for generating an averaged signal by averaging the voltage of the motor supply signal. A third means is coupled to an output of the first means for generating a signal indicative of the change in motor current. A fourth means is coupled to an output of the second means for generating a speed error signal having a voltage level indicative of the difference in voltage between the voltage level of the averaged signal of the second means and a reference voltage. A fifth means turns on the first means periodically, and turns off the first means following a delay corresponding to the difference between the value of the speed error signal and the value of the change in motor current signal. These means provide a substantially constant average motor supply voltage level resulting in a substantially constant motor speed approximately equal to a desired motor speed, and a substantially symmetrical motor current supply signal waveform.
According to yet another aspect of the present invention, a method of controlling the rotational speed of a brushless DC motor is provided. A motor supply signal is switchably passed from an electrical power source to a brushless DC motor. The voltage level of the motor supply signal is averaged to form an averaged signal. An error signal is generated having a voltage level indicative of the difference in voltage between the averaged signal and a reference voltage. A motor current signal is generated having a voltage level indicative of the change in current of the motor supply signal. The motor supply signal is modulated in response to the difference in value between the error signal and the motor current signal in order to provide a substantially constant voltage level and a substantially symmetrical motor current waveform.
According to another aspect, the present invention is directed to a method of regulating the rotational speed of a DC brushless motor, wherein the motor has at least one input terminal, a plurality of motor windings electrically coupled to the at least one input terminal, and a rotor defining a rotor mass and electromagnetically coupled to the motor windings and rotatable by interaction with an electromagnetic field generated by current flowing through the motor windings. The method comprises the following steps:
(i) passing motor supply signals from an electrical power source through an electrical conduction switch having an input, an output, and a control terminal;
(ii) pulse width modulating the motor supply signals passing through the electrical conduction switch and generating pulse width modulated motor supply signals;
(iii) transmitting the pulse width modulated motor supply signals through a substantially inductorless path extending from the output of the electrical conduction switch to the at least one input terminal;
(iv) using the windings of the motor to integrate the pulse width modulated motor supply signals and function as an inductor filter to smooth the current level of the motor supply signals; and
(v) using the rotor mass as a filter capacitor to smooth the voltage level of the motor supply signals.
According to yet another aspect, the present invention is directed to a control circuit for regulating the rotational speed of a DC brushless motor, wherein the motor has at least one input terminal, a plurality of motor windings electrically coupled to the at least one input terminal, and a rotor defining a rotor mass and electromagnetically coupled to the motor windings and rotatable by interaction with an electromagnetic field generated by current flowing through the motor windings. The control circuit comprises an electrical conduction switch having an input, an output, and a control terminal for passing a motor supply signal to the DC brushless motor from a voltage across first and second terminals of a DC voltage source. The input terminal of the switch is coupled to the first terminal of the DC voltage source, and the output terminal of the switch is coupled to the at least one input terminal of the motor. A substantially inductorless path extends from the output of the electrical conduction switch to the at least one input terminal of the motor. A pulse width modulator has an output terminal coupled to the control terminal of the electrical conduction switch for pulse width modulating the motor supply signals passing through the electrical conduction switch and generating pulse width modulated motor supply signals. The pulse width modulated motor supply signals are transmitted through the substantially inductorless path extending from the output of the electrical conduction switch to the at least one input terminal. The windings of the motor integrate the pulse width modulated motor supply signals and function as an inductor filter to smooth the current level of the motor supply signals. And the rotor mass functions as a filter capacitor to smooth the voltage level of the motor supply signals.
One advantage of the preferred embodiments of the present invention is that the motor voltage signal is compared against the reference voltage to generate an error signal, and the error signal is in turn compared against the motor current signal to pulse width modulate the motor input signal. Accordingly, the preferred embodiments of the apparatus and method of the present invention employ both a voltage feedback loop, and a current feedback loop embedded within the voltage feedback loop to maintain a substantially constant motor speed over a wide range of power supply voltages, to accurately select and control motor speed, and to do so while maintaining a substantially symmetrical armature current waveform.